Conventional bone fixation elements, such as pedicle screws, include a bone anchor retained within an anchor seat and captured by a collet. Pedicle screw assemblies include a plurality of pedicle screws joined by a rod that extends through rod slots formed in the pedicle screws. Uni-planar pedicle screws provide one degree of freedom. That is, the bone anchor retained within the anchor seat and, in some systems, the collet, is free to move with respect to the anchor seat in only one plane, e.g., the sagittal plane. Motion of the bone anchor is limited to this sagittal plane in conventional pedicle screws by a pinning or staking process during manufacture of the assemblies to create a pivot in the sagittal plane. As a result, the height of the bone screw is limited by the orientation of the rod slot. Unfortunately, if the rod slot isn't in line with the trajectory of the rod, the anchor seat must be turned, which results in either advancing or withdrawing the screw toward and away from the bone surface.
It is therefore desirable to provide a bone fixation element that allows the screw head to angulate in a desired plane while also allowing the bone anchor to rotate freely with respect to the anchor seat without advancing or withdrawing the screw toward or away from the bone surface.